Calibration of force based touch panel systems

ABSTRACT

A method and system are provided to correct inaccuracies in touch location determination associated with mechanical distortion of the touch screen. Calibration parameters are provided for a touch screen characterizing an error in an expected touch signal associated with mechanical distortion of the touch screen. A force responsive touch signal having the error is detected and the touch location determined using the calibration parameters to correct the error in the touch signal. The calibration parameters are determined by applying mechanical distortion to the touch screen and characterizing the touch signal error associated with the mechanical distortion. The calibration parameters are produced using the characterization of the touch signal error.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 10/147,604, filed May17, 2002, now allowed, the disclosure of which is incorporated byreference in its entirety herein.

FIELD OF THE INVENTION

The present invention is directed generally to a touch sensing system,and more particularly to a method and system for calibrating a touchscreen system for more accurate determination of the location of a touchon the touch screen.

BACKGROUND

A touch screen offers a simple, intuitive interface to a computer orother data processing device. Rather than using a keyboard to type indata, a user can transfer information through a touch screen by touchingan icon or by writing or drawing on a screen. Touch screens are used ina variety of information processing applications. Transparent touchscreens are particularly useful for applications such as cellphones,personal data assistants (PDAs), and handheld or laptop computers.

Various methods have been used to determine touch location, includingcapacitive, resistive, acoustic and infrared techniques. Touch locationmay also be determined by sensing the force of the touch through forcesensors coupled to a touch surface. Touch screens that operate bysensing touch force have several advantages over other technologiesmentioned above. First, force sensors do not require the touch surfaceto be composed of special materials that may inhibit opticaltransmission through the touch surface, as in a resistive touch sensor.

Further, force sensors do not rely on a lossy electrical connection toground, as is required by a capacitive touch screen, and can be operatedby a finger touch, gloved hand, fingernail or other nonconductive touchinstrument. Unlike surface acoustic wave technology, force sensors arerelatively immune to accumulations of dirt, dust, or liquids on thetouch surface. Finally, a force sensor is less likely to detect a closeencounter with the touch surface as an actual touch, which is a commonproblem with infrared touch screens.

A force based touch screen may be built with a minimum of three forcesensors spaced in a triangular pattern under a touch surface. Such anarrangement may provide signals sufficient to determine the netperpendicular force and the two moments necessary to compute touchlocation. Touch screen devices also may be built with a larger number ofsensors. Commonly, four corner sensors may be used, in part to harmonizewith the symmetry of the rectangular touch surface typically required.Upon application of a touch, the forces sensed by the touch screensensors may be used to determine the touch location. However,determination of the touch location may be affected by a number offactors in addition to the touch force. Twisting, squeezing or otherwisedistorting the touch screen during a touch may cause inaccuracies in thetouch location determination.

SUMMARY OF THE INVENTION

In general terms, the present invention relates to a method and systemfor detecting the location of a touch on a touch sensor. Features of thepresent invention are particularly useful when combined with amicroprocessor-based system operating a display device enhanced by atransparent touch screen.

In accordance with one embodiment of the present invention, a method fordetermining a touch location on a touch screen is provided. The touchscreen is defined by a plurality of touch sensors disposed to measure asignal indicative of a touch force component that is perpendicular to atouch surface. The method includes providing calibration parameters forthe touch screen acquired using the touch sensors and the touch surface.The calibration parameters characterize an error in an expected touchsignal associated with mechanical distortion of the touch screen. Aforce responsive touch signal having the error is detected and touchlocation determined using the calibration parameters to compensate forthe error.

In another embodiment of the present invention, a method for calibratinga touch screen includes applying a mechanical distortion to the touchscreen and detecting a force responsive touch signal arising from themechanical distortion of the touch screen. Touch signal error associatedwith the mechanical distortion is characterized and calibrationparameters are produced using the characterization of the touch signalerror.

In accordance with a further embodiment of the present invention, atouch screen system includes a touch surface, a plurality of forceresponsive touch sensors mechanically coupled to the touch surface andproducing a sensor signal in response to a touch applied to the touchsurface, and a control system couple to the touch sensors and receivingthe sensor signals. The control system is configured to providecalibration parameters for the touch screen acquired using the touchsensors and the touch surface. The calibration parameters characterizean error in an expected touch signal associated with mechanicaldistortion of the touch screen. The control system detects a forceresponsive touch signal having the error and determines a touch locationusing the calibration parameters to compensate for the error in thetouch signal.

In yet another embodiment of the present invention, a touch screendisplay system includes a touch surface, a plurality of touch sensors, acontrol system and a display for displaying information through thetouch screen system. The control system is configured to providecalibration parameters for the touch screen acquired using the touchsensors and the touch surface. The calibration parameters characterizean error in an expected touch signal associated with mechanicaldistortion of the touch screen. The control system detects a forceresponsive touch signal having the error and determines a touch locationusing the calibration parameters to compensate for the error in thetouch signal.

In another embodiment of the present invention, a touch screencalibration system comprises a mechanical distortion system for applyingmechanical distortion to the touch screen, a detection system fordetecting force responsive sensor signals arising from the mechanicaldistortion, and a processor coupled to the detection system. Theprocessor is configured to detect a force responsive touch signalarising from the mechanical distortion of the touch screen andcharacterize a touch signal error associated with the mechanicaldistortion of the touch screen. The processor is further configured toproduce calibration parameters using the characterization of the touchsignal error.

A further embodiment of the present invention includes a system fordetermining a touch location on a touch screen. The touch screen isdefined by a plurality of touch sensors mechanically coupled to a touchsurface. The system includes means for providing touch screencalibration parameters acquired using the touch surface and the touchsensors, means for detecting a touch signal having the touch signalerror, means for correcting the touch signal using the touch screencalibration, and means for determining the touch location using thecorrected touch signal. The touch screen calibration parameterscharacterize a touch signal error associated with a mechanicaldistortion of the touch screen affecting a touch signal.

In another embodiment of the present invention, a system for calibratinga touch screen is provided. The system includes means for applyingmechanical distortion to the touch screen, means for detecting sensorsignals associated with the mechanical distortion, and means forcalibrating the touch screen to compensate for the mechanicaldistortion.

In a further embodiment of the present invention, a computer-readablemedium is configured with executable instructions for causing one ormore computers to perform a method for determining a touch location on atouch screen. The touch screen defined by a touch surface and aplurality of touch sensors disposed to measure a signal indicative of atouch force component that is perpendicular to the touch screen. Themethod for determining touch location includes providing calibrationparameters for the touch screen acquired using the touch sensors and thetouch surface, the calibration parameters characterizing an error in anexpected touch signal associated with mechanical distortion of the touchscreen, detecting a force responsive touch signal having the error; anddetermining the touch location using the calibration parameters tocompensate for the error in the touch signal.

Yet another embodiment of the present invention includes acomputer-readable medium configured with executable instructions forcausing one or more computers to perform a method of calibrating a touchscreen. The method comprises applying mechanical distortion to the touchscreen, detecting a force responsive touch signal arising from themechanical distortion of the touch screen, characterizing a touch signalerror associated with the mechanical distortion, the touch signal errorarising in a force responsive touch signal, and producing calibrationparameters using the characterization of the touch signal error.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The figures and the detailed description which follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 schematically illustrates a perspective view of a touch screenwith force sensors located at the corners of the touch screen inaccordance with an embodiment of the invention;

FIG. 2 schematically illustrates a cross-sectional view of a capacitiveforce sensor in accordance with an embodiment of the invention;

FIG. 3 is a block diagram of a touch screen and touch screen controlsystem in accordance with an embodiment of the invention;

FIG. 4 schematically illustrates a touch screen under torsion;

FIG. 5 is a flowchart conceptually illustrating a method for determiningtouch location using a characterization of an error associated withdistortion of the touch screen in accordance with an embodiment of theinvention;

FIG. 6 is a flowchart conceptually illustrating a method forcharacterizing an error caused by distortion of the touch screen inaccordance with an embodiment of the invention;

FIG. 7 is a more detailed flowchart conceptually illustrating a methodfor characterizing an error caused by distortion of the touch screen inaccordance with an embodiment of the invention;

FIGS. 8A and 8B schematically illustrate a method of applying twodifferent support strain configurations to a touch screen in accordancewith an embodiment of the invention;

FIGS. 9A and 9B schematically illustrate another method of applying twodifferent support strain configurations to a touch screen in accordancewith an embodiment of the invention;

FIG. 10 is a flowchart of a method of determining a basic calibration ofthe touch screen in accordance with an embodiment of the invention;

FIG. 11 is a flowchart conceptually illustrating a method forcharacterizing an error caused by distortion of the touch screencomputed in a single step from data responsive to both known forces anddeliberately applied distortions in accordance with an embodiment of theinvention;

FIG. 12 is a block diagram of a touch screen calibration system inaccordance with an embodiment of the invention;

FIG. 13 is a block diagram of a data processing system using a touchsensing interface in accordance with an embodiment of the invention;

FIG. 14 illustrates a touch screen controller in accordance with anembodiment of the invention; and

FIG. 15 illustrates a touch screen calibration system in accordance withan embodiment of the invention.

The invention is amenable to various modifications and alternativeforms. Specific embodiments of the invention have been shown by way ofexample in the drawings and will be described in detail. It should beunderstood, however, that the intention is not to limit the invention tothe particular embodiments described. On the contrary, the intention isto cover all modifications, equivalents, and alternatives falling withinthe scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings which form a part hereof, andvarious embodiments by which the invention may be practiced are shown byway of illustration. It is to be understood that other embodiments maybe utilized, and structural and functional changes may be made withoutdeparting from the scope of the present invention.

As stated above, and for other reasons stated below which will becomeapparent upon reading the present specification, there is a need for amethod and a system for accurately determining the location of a fingertouch or an instrument touch on a touch surface. There exists a furtherneed for such a method and system that calculates touch location withcorrection for mechanical distortions applied to the touch screen duringthe time in which the touch location information is obtained todetermine touch location.

The present invention is applicable to touch sensing techniques and isbelieved to be useful when features of the present invention arecombined with a data processing system operating a display deviceenhanced by a transparent touch screen. For example, a touch screen ofthe present invention may be used in a desktop, handheld or laptopcomputer system, a point-of-sale terminal, personal data assistant(PDA), or a cell phone. Although described in combination with amicroprocessor-based system, a touch screen device of the presentinvention may be combined with any logic-based system, if desired.

The present invention provides for the accurate determination of a touchlocation on a force based touch screen in the presence of mechanicaldistortions of the touch screen. A touch may be sensed by a number oftouch sensors and represented by one or more touch signals. Accuratetouch location determination involves measuring the magnitudes of one ormore touch signals during a touch on the touch screen. At the time thetouch information is obtained to determine the touch location, the touchscreen may be influenced by a number of factors, such as those caused byan operator twisting or squeezing the touch screen device. Suchdisturbances of the touch screen during a time the touch signal is beingprocessed to determine the touch location may lead to inaccuracies inthe calculated touch location.

A perspective view of a rectangular touch screen is schematicallyillustrated in FIG. 1. A touch surface 100 is shown disposed proximateto force sensors located at respective corners of the touch surface 100.The touch surface 100 and force sensors 110, 120, 130, 140 are locatedwithin a touch screen housing (not shown).

As a stylus, finger or other touching device 152 presses the touchsurface 100, a touch force 155 is exerted upon the touch surface 100 atthe touch location 150. The touch force 155 creates forces F1, F2, F3,F4 on the force sensors 110, 120, 130, 140 perpendicular to the touchsurface 100. The force sensors 110, 120, 130, 140 may be driven with analternating electrical signal. The perpendicular forces F1, F2, F3, F4cause a change in the capacitance of the force sensors 110, 120, 130,140, thereby causing the signal coupled through the force sensors 110,120, 130, 140 to change. The force responsive signals derived from theforce sensors 110, 120, 130, 140 may be used to calculate touchlocation. Although the touch screen illustrated in FIG. 1 is rectangularwith sensors located at the corners, various configurations using threeor more touch sensors with differing touch surface shapes may also beused.

The sensors 110, 120, 130, 140, may be, for example, small capacitiveforce sensors constructed of two capacitor plates separated by a gap. Acapacitive force sensor may be arranged so that when a touch force ofsufficient magnitude and direction is applied to the touch surface, onecapacitor plate deflects towards the second plate. The deflection altersthe distance between the capacitor plates, changing the capacitance ofthe sensor. The touch force may be measured by control system circuitryas a change in an alternating electrical signal applied to the touchsensor. One embodiment of a capacitive force sensor appropriate for usein touch screen applications is described in co-owned U.S. PatentApplication, U.S. Ser. No. 09/835,040, filed Apr. 13, 2001, entitled“Method and Apparatus for Force-Based Touch Input” (US publicationnumber 02-0149571-A1, published Oct. 17, 2002), which is herebyincorporated herein by reference.

A force sensor is appropriate for use with a liquid crystal display(LCD), cathode ray tube (CRT) or other electronic display, and isschematically illustrated in FIG. 2. In this particular embodiment, thesensor measures the applied force based on the change of capacitance ofa capacitive element. A touch surface 210, or overlay, is located withina structure or housing 215. The touch surface 210 is typicallytransparent to allow viewing of a display or other object through thetouch surface. In other applications, the touch surface 210 can beopaque.

The structure or housing 215 may be provided with a large centralaperture through which the display may be viewed. If desired, theundersurface of the housing 215 may be seated directly against thesurface of such a display, over the border surrounding its active area.In another embodiment, as mentioned above, the overlay may be replacedby a structure including a display unit, such as an LCD.

A capacitive sensor 220 may be positioned between the touch surface 210and the housing 215. An interconnect 225, with attachment lands 233, maybe coupled to the housing 215 by soldering, cementing, or by othermethods. A conductive area forms a first conductive element 234 on theinterconnect 225. A second conductive element 235 with a centralprotrusion 240, for example a dimple, may be attached to the lands 233of the interconnect 225 by soldering, for example. A small gap 280 isformed between the first conductive element 234 and the secondconductive element 235, either by the shape of the second conductiveelement 235, or by the process of attaching the second conductiveelement 235 to the interconnect 225. The width of the gap 280 may beapproximately 1 mil, for example. A capacitor is formed by theconductive elements 234, 235 separated by the gap 280.

An optional bearing surface 270 may be interposed between the touchsurface 210 and the second conductive element 235. This may protect theunderside of touch surface from indentation or from damage by theprotrusion 240, especially in cases where the overlay is made of softermaterial. The bearing surface 270 may also mount to the touch surface210 through a thin layer (not shown) of elastomer or of highly pliableadhesive, thereby providing a lateral softening function. It will beappreciated that, in normal operation, the touch surface 210 or bearingsurface 270 is in contact with the protrusion 240: these elements areshown separated only for clarity in the illustration.

The second conductive element 235 combines the functions of a spring anda capacitor plate. As a perpendicular force is applied to the touchsurface 210, the second conductive element 235 flexes, decreasing thewidth of the gap 280 and increasing the capacitance of the sensor 220.This change in capacitance may be measured and related to the forceapplied to the touch surface 210. Although a touch screen usingcapacitive force sensors is described, other types of force sensors maybe used in a similar manner, including, for example, piezoelectricsensors and strain gauge sensors.

One of the advantages of a force-based touch screen is that the numberof optically distinct layers positioned between the display unit and theuser is low. Typically, the overlay positioned over the display unit isa single layer of glass or relatively stiff polymer, for examplepolycarbonate or the like, which may be chosen for suitable opticalqualities. This contrasts with other types of touch screen, such asresistive or capacitive touch screens, that require several, potentiallyoptically lossy, layers over the display unit. The electricallyconductive thin films required in resistive or capacitive touch screenstypically have a high index of refraction, leading to increasedreflective losses at the interface. This is a particular problem inresistive screens where there are additional solid/air interfaces andwhere antireflection coatings are not useful, since the conductivelayers must be able to make physical contact. A screen overlay for aforce-based touch screen, however, has only its upper and lowersurfaces; these may be treated to reduce reflective losses and to reduceglare. For example, the overlay may be provided with matte surfaces toreduce specular reflection, and/or may be provided with anti-reflectioncoatings to reduce reflective losses.

Touch signals representing the force of a touch acting on the touchscreen are produced by one or more touch sensors coupled to a touchsurface of the touch screen. A touch signal may be derived from a singlesensor, or by combining sensor signals from two or more touch sensors.Determination of a touch location involves analyzing the sensor signalsproduced by the touch sensors. A tap touch in a single locationcharacteristically produces a touch signal that increases in magnitudeas the touch is applied and then decreases in magnitude as the touch isremoved. A touch may be a continuing touch wherein the touch remains onthe touch surface for a period of time. For example, the touch may bepresent in a single location for a period of time. Further, the touchmay be a “streaming touch,” wherein the touch is applied at onelocation, moved across the surface of the touch screen, and removed atanother location, causing the generation of a continuously changingsignal at each sensor.

Calculation of the touch location at any time, t, may be performed, forexample in a four sensor screen, using combinations of the forceresponsive sensor signals f₁(t), f₂(t), f₃(t), f₄(t). The forceresponsive signals generated by the touch sensors may be used tocalculate various touch signals, including the moment about the y-axis,M_(Y)(t), moment about the x-axis, M_(x)(t), and the total z-directionforce, F_(Z)(t). The coordinates of the touch location may be determinedfrom the touch sensor signals, as provided in Equation 1. Assuming areference point in the center of the touch screen, a perfectly rigidtouch surface, ideal conditions, with no errors, background fluctuationsor disturbances present other than the touch force. The force andmoments employed in Equation 1 may be evaluated as in Equation 1 b.$\begin{matrix}{{{X(t)} = \frac{M_{Y}(t)}{F_{Z}(t)}}{{Y(t)} = \frac{M_{X}(t)}{F_{Z}(t)}}} & \lbrack 1\rbrack\end{matrix}$

where, for this particular case,M _(X)(t)=(f ₁(t)+f ₂(t))−(f ₃(t)+f ₄(t));M _(Y)(t)=(f ₂(t)+f ₄(t))−(f ₁(t)+f ₃(t)); andF _(Z)(t)=f ₁(t)+f ₂(t)+f ₃(t)+f ₄(t).  [1b]

The sensor signals are directed to a control system that determines atouch location from the force responsive sensor signals. FIG. 3schematically illustrates a block diagram of a touch screen 300 andtouch screen control system 350 arranged in functional blocks inaccordance with the principles of the present invention. It will beappreciated that there exist many possible configurations in which thesefunctional blocks may be arranged. The example depicted in FIG. 3 is onepossible functional arrangement.

In the exemplary embodiment illustrated in FIG. 3, a touch surface 305is configured proximate to four force sensors 301, 302, 303, 304arranged at the respective corners of the touch surface 305. The touchsurface 305 and force sensors 301, 302, 303, 304 are arranged in a touchscreen housing (not shown). The sensors 301, 302, 303, 304 may be chosenfrom a variety of sensing technologies, including capacitive,piezoelectric and strain gauge sensors. The sensors 301, 302, 303, 304measure the force of a touch detected at the sensor locations and arecoupled to drive/sense circuitry 310, 320, 330, 340 located within thecontrol system 350. Alternatively, some components of the drive/sensecircuitry may be located near the corresponding sensor. An energizingsignal developed in the drive circuitry 312, 322, 332, 342 for eachsensor is used to energize the sensors 301, 302, 303, 304. Each sensor301, 302, 303, 304 produces a touch force signal corresponding to atouch force applied to the sensor through the touch surface 305. Thetouch force signal developed by each sensor 301, 302, 303, 304 isdetected by sense circuitry 311, 321, 331, 341 located within thecontrol system 350.

Analog voltages representing the touch force at each sensor location areproduced by the sense circuitry 311, 321, 331, 341. The analog voltagesare sampled and multiplexed by the sampling circuitry 360 at a ratesufficient to acquire an adequate representation of the force responsivesensor signals for determining touch presence and location. The sampledsignals are digitized by an analog to digital (A/D) converter 370. Thedigitized sensor signals are directed to processor circuitry 380. Theprocessor circuitry 380 performs calculations to determine a touchlocation. The processor circuitry 380 may also include filteringcircuitry 382 for signal conditioning and memory circuitry 386 forstorage of touch signal values. The processor circuitry 380 may alsoinclude one or more timers 384 for determining various interval anddelay timing of the touch signal associated with determination of thepreferred time for making the touch location measurement. The processorcircuitry 380 may perform a number of additional control systemfunctions, including controlling the touch signal sampling circuitry360, the multiplexer circuitry 360, and the A/D converter 370.

It may be found advantageous to implement the touch screen controlsystem 350, or its equivalent, on a single mixed-mode integrated circuitchip. In such an implementation, it may be advantageous to replacesampling circuitry 360 and A/D converter 370 with a set of delta-sigmaconverters operating in parallel, one for each signal channel.

Consider a force touch screen in a thin portable device, such as a PDA.Take the case to be one wherein the device is roughly rectangular inoutline, with a rectangular touch surface supported by four forcesensors near the corners as depicted in FIG. 4. While one hand holds astylus to apply touches to the surface, the opposite hand may be holdingthe device against a table, or grasping it free of any surface support.Uneven pressure from this opposite hand may serve to twist theenclosure, leading to a pattern of forces, such as, for example, F1, F2,F3, and F4 applied to the enclosure at or near its corners. This causesa torsional distortion of the touch screen device, such that a linealong one edge 410 of the device tends to move very slightly out ofparallel with an opposing edge 420. The touch surface will, to somedegree, resist following this distortion of the support structure of thetouch screen, such that the forces in one diagonally opposing pair offorce sensors, located at diagonally opposing corners 402 and 403 becomeless positive (or more negative), while the forces located at corners401 and 404 change by an equal amount in the opposite sense.

While the grasping or restraining hand applies a force of perhaps 100grams downward on opposing device corners 402, 403, equilibrium may besustained by equal upward reaction forces at the other two corners 401,404. Since a substantial portion of the entire device stiffness mayreside in the touch overlay, the torsional force applied to it throughthe sensors may be a substantial portion of the total applied to thedevice. If this portion is one-quarter, for instance, then the sensorsin this example may each experience + or −25 grams of torsionallyapplied force. At the same time, a 20-gram touch force applied to thecenter of the screen should appear as a 5-gram addition to each sensor.The force from the restraining hand may fluctuate rapidly. The signalresulting from the touch force, indicating the touch location, then, maybe several times smaller than a simultaneously present fluctuatinginterference.

In a second example, consider a public-access display equipped with arobust, vandal-resistant touch screen. Such a screen may be very thickand rigid. The weight of moving equipment, the pressure of wind on thebuilding, or even the weight of passing footsteps may cause smalltorsional distortions in a kiosk-type or wall mounted enclosure. Theresult may again be a significant and varying torsional force patternapplied to the sensors.

Difficulties of the sort just discussed may arise whenever a force-basedtouch location is to be derived from a surface supported on more thanthree sensors. Although touch location may be determined using a minimumof three force sensors, certain advantages may mandate the use of alarger number. Additional points of support for the touch surface, forexample, may prevent it from flexing excessively in response to touchforces. Conversely, to the extent that the support structure beneath thetouch screen flexes, sensor connections beyond the third sensor mayserve to constrain the touch surface to flex in concert. Both of theseeffects reduce relative motion between the edges of the touch surfaceand any surrounding frame or bezel, and between the touch surfacestructure in general and the structures below. There may be seals,preload springs, or other connections running between the touch surfaceand the larger device that shunt varying forces around the force sensorsin response to such relative motion. As these unmeasured shunt forcesmay lead to errors in force location, their reduction can beadvantageous. Reduction is achieved, however, by passing forces tendingto distort one structure, into another through the force sensors. Thismay itself become a source of inaccuracies.

With three force sensors, each combination of position and perpendiculartouch force may be associated with a specific set of sensor values. Whenmore than three sensing connections support a touch surface structure,however, there is no longer a one-to-one relationship between touchlocations and sensor response patterns. The division of perpendiculartouch force among more than three sensors is a statically indeterminateproblem, and the many different possible response patterns for a giventouch are seen to result from different patterns of device strain.

In an idealized situation where the support structure and touch screenare perfectly rigid, strain patterns within the touch surface structureand support structure remain constant throughout the course of a touch.In this idealized situation of perfect rigidity, the change in sensoroutputs from a moment before a touch to one during it would becharacterized by a single fixed pattern as with the case of only threesensors. In this situation, the relative magnitudes of the differentsensor output changes would depend only on the location touched. Touchlocation may be computed from such pre-touch to during-touch changes.Assuming, then, that forces from internal strain are not excessive forthe force sensors, it is seen that the assumption of perfect rigiditysimplifies the calculation of touch location.

Slightly flexible structures may, however, be more practical or morecost effective. Such flexible structures may change strain patternduring the course of a touch, due either to the stress of the touchforce itself, or due to independently changing stresses applied to thesupport structure.

In an ideally calibrated force-touch screen with somewhat flexiblestructure, the total force signal and two moment signals needed fortouch location computation are formed from precise linear combinationsof the sensor outputs, these combinations having the property ofcanceling exactly to report zero total force and moment values forpatterns of perpendicular sensor force arising from indeterminacy anddevice flexure. The coefficients employed for such combinations, atwhatever level of accuracy achieved, may be termed a “calibration” forthe touch screen in question. A fully accurate calibration may reflectthe exact locations and sensitivities of the force sensors, along withany electronic sensitivities or cross-talk. Imprecise calibration valuesmay lead to location errors. Those resulting from varying mechanicaldistortion of the touch screen may be unexpectedly large, and maybenefit from special attention in the calibration process. For clarityin the discussion below, a calibration prepared without specialattention to potential inaccuracies from distortion will be termed a“basic calibration”.

For a force-based touch screen with n sensors, Let vector {right arrowover (F)}(t) represent the set of all sensor values at time t collectedtogether into a list in a predetermined order:{right arrow over (F)}(t)=[f ₁(t), . . . f _(n)(t)]  [2]

The variable t may be taken to be the continuous-valued time, or thediscrete-valued sample number, to which the data correspond, asconvenient.

The coefficients of combination comprising the calibration may also becollected together into vectors {right arrow over (C)}_(Z), {right arrowover (C)}_(Y), and {right arrow over (C)}_(X):{right arrow over (C)}_(Z)=[c_(Z1), . . . c_(Zn)]{right arrow over (C)}_(Y)=[c_(Y1), . . . c_(Yn)]{right arrow over (C)}_(X)=[c_(X1), . . . c_(Xn)]  [3]

These associate the proper weights with the sensor channels, such that:F _(Z)(t)={right arrow over (C)} _(Z) ·{right arrow over (F)}( t)M _(Y)(t)={right arrow over (C)} _(Y) {right arrow over (F)}( t)M _(X)(t)={right arrow over (C)} _(X) ·{right arrow over (F)}( t)  [4]

where F_(Z)(t) represents the perpendicular component of total touchforce, M_(Y)(t) represents the moment of the touch force about thedesired Y-axis, and M_(X)(t) represents the moment of the touch forceabout the desired X-axis. The desired axes in question are those of thatcoordinate grid, lying in the touch plane, with respect to which touchlocation is to be reported. The units in which M_(Y)(t) and M_(X)(t) arerepresented may be any convenient choice, and may be different for thetwo axes. In particular, they may be chosen such that final touchcoordinates may be computed directly using Equation 1, repeated belowfor convenience: $\begin{matrix}{{{X(t)} = \frac{M_{Y}(t)}{F_{Z}(t)}}{{Y(t)} = \frac{M_{X}(t)}{F_{Z}(t)}}} & \lbrack 1\rbrack\end{matrix}$

These coordinates may normally be calculated and reported only at timeswhen the magnitude of F_(Z)(t) is such as to indicate the presence of adeliberate touch that is strong enough to be accurately located. Onlyone touch location may be reported, or many successive locations may bereported for a continuing touch.

When there are more than three sensors, the vectors {right arrow over(C)}_(Z), {right arrow over (C)}_(Y), and {right arrow over (C)}_(X)allow for n−3 additional vectors mutually orthogonal to these and toeach other. These additional vectors may be added in arbitraryproportion to an existing set of sensor outputs without changing acomputed touch location. Furthermore, if the calibration vectors areperfectly accurate, these additional vectors may correspond to distinctpatterns of perpendicular sensor force associated with staticindeterminacy, whereby such indeterminacy forces need not cause error.In particular, when n=4, the single such vector may correspond tooverall torsional flexure of the device. However, if the calibrationvectors are not perfectly accurate, this one orthogonal vector may notexactly match the sensor output from torsion. Then fluctuating torsion,especially arising from potentially large forces applied to the supportstructure, may lead to location errors.

One aspect of the present invention is directed to reducing the effectof mechanical distortions of the touch screen, such as torsion, on thedetermination of the touch location on the touch screen. Mechanicaldistortions of the touch screen may arise from the exemplary situationsdiscussed above, or from other mechanical distortions affecting theaccuracy of the touch location measurement. FIG. 5 illustrates, in broadand general terms, a method of reducing the effect of mechanicaldistortion of the touch screen to increase touch location accuracy.Calibration parameters acquired using the touch surface and the touchsensors are provided 510. The calibration parameters characterize anerror in an expected touch signal associated with mechanical distortionof the touch screen. A touch signal having the error is detected 520.The touch location is determined using the calibration parameters tocompensate for the error in the touch signal 530.

Another aspect of present invention is directed to a method and systemfor characterizing the effect of mechanical distortions on the touchsignal. FIG. 6 illustrates, in broad and general terms, a method fordetermining calibration parameters characterizing the effect ofmechanical distortion on the touch screen. One or more deliberatemechanical distortions are applied to the touch screen 610. The forceresponsive touch signals arising from the mechanical distortion of thetouch screen are detected 620. The touch signal error associated withthe mechanical distortion is characterized 630. Calibration parametersare produced using the characterization of the touch signal error 640.

In accordance with one approach, a method for characterizing the errorassociated with touch screen torsion is conceptually illustrated in theflowchart of FIG. 7. A basic calibration may be obtained in addition tothe characterization of the mechanical distortion. The basic calibrationmay be calculated from either the nominal sensor locations andsensitivities of the touch screen design, or from sensor locations andsensitivities measured on a unit by unit basis.

Following basic calibration, two sets of force sensor output values areaccumulated, corresponding to two different states of torsion. Both areaccumulated while no force is externally applied to the touch surface.The differences formed by subtracting the second set of sensor outputvalues from the first may then be normalized to a vector of unitmagnitude, and the result taken to be the normalized response vector totorsion. Thus, for contrasting sets of values taken at times t_(Q1) andt_(Q2), the torsion response vector, {right arrow over (F)}_(Q), may begiven by: $\begin{matrix}\begin{matrix}{{\overset{\rightharpoonup}{F}}_{Q} = \left\lbrack {f_{Q\quad 1},f_{Q\quad 2},f_{Q\quad 3},f_{Q\quad 4}} \right\rbrack} \\{= {\left\lbrack {{f_{1}\left( t_{Q\quad 2} \right)},{f_{2}\left( t_{Q\quad 2} \right)},{f_{3}\left( t_{Q\quad 2} \right)},{f_{4}\left( t_{Q\quad 2} \right)}} \right\rbrack -}} \\{\left\lbrack {{f_{1}\left( t_{Q\quad 1} \right)},{f_{2}\left( t_{Q\quad 1} \right)},{f_{3}\left( t_{Q\quad 1} \right)},{f_{4}\left( t_{Q\quad 1} \right)}} \right\rbrack}\end{matrix} & \lbrack 5\rbrack\end{matrix}$and a parallel vector of unit length may be given by: $\begin{matrix}{{{{\overset{\rightharpoonup}{F}}_{QN} = \frac{{\overset{\rightharpoonup}{F}}_{Q}}{\sqrt{{\overset{\rightharpoonup}{F}}_{Q} \cdot {\overset{\rightharpoonup}{F}}_{Q}}}},{whereby}}{{{\overset{\rightharpoonup}{F}}_{QN} \cdot {\overset{\rightharpoonup}{F}}_{QN}} = 1.}} & \lbrack 6\rbrack\end{matrix}$

More particularly, an embodiment of a method for the collection oftorsion-responsive sensor data may be described as conceptuallyillustrated in the flowchart of FIG. 7 and the touch screen diagrams ofFIGS. 8A-B and FIGS. 9A-B. Following a determination of the basiccalibration of the touch screen 710, a first degree of deliberatemechanical distortion may be applied to the touch screen 720. A firstset of sensor response values may be measured with the first degree oftorsion applied 730 and with no touch or other force applied to thetouch surface. In one example, the first degree of torsion may simply bea condition of zero torsion, as illustrated in FIG. 8A. The first set ofsensor response values is taken from an unstressed touch surface 800where touch sensors located at corners 801, 802, 803 804 experience noperpendicular force or mechanical distortion of the touch surface 800.

A second deliberate torsion may then be imposed on the touch screen 740,and a second set of force sensor outputs measured 750, again while noforce is externally applied to the touch surface. A satisfactorydeliberate distortion may be achieved, for example, by an apparatus thatapplies an upward force under one corner 803 of the device, asillustrated in FIG. 8B. while the other three corners 801, 802, 804 areheld stationary. Alternatively, the first set of sensor response valuesmay also be measured with a deliberate mechanical distortion applied,but with effect opposite to that of the second set.

In some configurations, the weight of the device itself is a sufficientsource of distorting force. In this configuration, illustrated in FIG.9, the touch sensors may experience the weight of the touch surface 900equally distributed to the sensors as forces f₁(t₁)=F1, f₂(t₁)=F2,f₃(t₁)=F3 and f₄(t₁)=F4 carried at corners 901, 902, 903, 904 of thetouch surface 900. A shim 950 may be inserted under one corner 904 ofthe touch screen to apply a first deliberate torsion to the touch screencorresponding to altered forces f₁(t₂)=F1′, f₂(t₂)=F2′, f₃(t₂)=F3′ andf₄(t₂)=F4′ at touch sensors located at corners 901, 902, 903, and 904,respectively. The shim 950 may simply be moved from under one corner tounder an adjacent corner to apply a second deliberate torsion to thetouch screen.

Turning back to FIG. 7, the difference between the two different sets offorce sensor output values, corresponding to two different states oftorsion are formed by subtracting the second set from the first 760 andthen normalizing the resultant vector 770 to a vector of unit magnitude.The resulting vector represents the normalized response vector totorsion. Because neither external force nor moment is present during theapplication of pure torsion, calibration vectors {right arrow over(C)}_(ZB), {right arrow over (C)}_(YB), and {right arrow over (C)}_(XB)acquired from the basic calibration should be orthogonal to thenormalized response vector to torsion:0 ?{right arrow over (C)} _(ZB) ·{right arrow over (F)} _(Q)0 ?{right arrow over (C)} _(YB) ·{right arrow over (F)} _(Q)0 ?{right arrow over (C)} _(XB) ·{right arrow over (F)} _(Q)   [7]

These conditions will hold only within the limits of the calibrationaccuracy. However, a torsion-corrected calibration {right arrow over(C)}_(ZT), {right arrow over (C)}_(YT), {right arrow over (C)}_(XT) maybe obtained from a basic calibration {right arrow over (C)}_(ZB), {rightarrow over (C)}_(YB), and {right arrow over (C)}_(XB) by taking each ofits vectors in turn, and removing any part parallel to the pure torsionresponse. This may be accomplished by subtracting an adjustment vectorfrom each basic calibration vector. The adjustment vector may in eachcase be formed 780 by multiplying the normalized response vector totorsion by its own dot product with the calibration vector in question:k _(QZ) ={right arrow over (C)} _(ZB) ·{right arrow over (F)} _(QN)k _(QY) ={right arrow over (C)} _(YB) ·{right arrow over (F)} _(QN)k _(QX) ={right arrow over (C)} _(XB) ·{right arrow over (F)} _(QN)  [8]The distortion corrected calibration vectors may then be determined 790by difference between the basic calibration and the product of theappropriate adjustment factor by the normalized response of the touchscreen to torsion.{right arrow over (C)} _(ZT) ={right arrow over (C)} _(ZB) −k _(QZ){right arrow over (F)} _(QN){right arrow over (C)} _(YT) ={right arrow over (C)} _(YB) −k _(QY){right arrow over (F)} _(QN){right arrow over (C)} _(XT) ={right arrow over (C)} _(XB) −k _(QX){right arrow over (F)} _(QN)  [9]

Before discussing further embodiments of the method of the invention, itis appropriate to briefly consider certain methods for developing abasic calibration for a force touch screen. Subject to certainassumptions, it can be shown that a basic calculated calibration may beobtained from:{right arrow over (C)}_(ZB)=[s₁, . . . s_(n)]{right arrow over (C)}_(YB)=[s₁y₁, . . . s_(n)y_(n)]{right arrow over (C)}_(XB)=[s₁x₁, . . . s_(n)x_(n)]  [10]where x_(i), y_(i) is the location at which a touch force passes intothe i^(th) sensor, as measured in the desired output locationcoordinates, and where s_(i) scales and standardizes the sensitivity ofthe i^(th) sensor and its associated electronics. That is, if f_(test)_(—) _(i) is the change in sensor i output in response to a trueperpendicular sensor test force F_(test) _(—) _(i) passing through,then: $\begin{matrix}{s_{i} = \frac{F_{test\_ i}}{f_{test\_ i}}} & \lbrack 11\rbrack\end{matrix}$

The accuracy of such a directly calculated calibration may becompromised by certain factors. Among these may be inaccuracy in themeasurements of sensitivity or coupling position, the presence ofparallel paths for perpendicular force other than the sensors, and thepresence of significant channel-to-channel cross talk in the wiring orelectronics.

In addition, the method of obtaining a calculated calibration, as so fardescribed, makes no provision for especially low susceptibility totorsional error. This may be improved upon, however, by applying thetorsion response corrections as described above to the basic calibrationvectors, {right arrow over (C)}_(ZB), {right arrow over (C)}_(YB),{right arrow over (C)}_(XB), to achieve a torsion corrected calibrationvectors, {right arrow over (C)}_(ZT), {right arrow over (C)}_(YT),{right arrow over (C)}_(XT).

A example of a basic calibration and its nominal calculated value isconsidered below. With four sensors, the basic form is given by:{right arrow over (C)}_(ZB)=[s₁, s₂, s₃, s₄]{right arrow over (C)}_(YB)=[s₁y₁, s₂y₂, s₃y₃, s₄y₄]{right arrow over (C)}_(XB)=[s₁x₁, s₂x₂, s₃x₃, s₄x₄]  [12]Returning to FIG. 1, we assume that the four corner sensors areprecisely located, and have the exactly desired sensitivity, which wewill assume to be unity. We further assume that the desired touchcoordinate system should have its origin in the screen center, that Xand Y should each range from −1.00 to +1.00, and that the edges of thisrange should extend to the sensors. The upper left sensor is thenlocated by: [x₁,y₁]=[−1,+1], the upper right sensor by: [x₂,y₂]=[+1,+1],the lower left sensor by: [x₃,y₃]=[−1,−1], and the lower right sensorby: [x₄,y₄]=[+1,−1]. This yields:{right arrow over (C)}_(ZB) _(—) _(FIG1)=[1,1,1,1]{right arrow over (C)}_(YB) _(—) _(FIG1)=[1,1,−1,−1]{right arrow over (C)}_(XB) _(—) _(FIG1)=[−1,1,−1,1]  [13]With this, or with any other exactly rectangular array of equallysensitive sensors, the normalized response to torsion is given by:{right arrow over (F)}_(QN) _(—) _(FIG1)=[−½,½,½,−½],  [14]which is orthogonal to the nominal calibration vectors.

In another approach, basic calibration vectors may be calculated on aunit-by-unit basis from sets of sensor response values measured inresponse to test forces applied to the touch surface of each completedunit. This approach may be advantageously simple and accurate.Determination of the basic calibration vectors by this method isconceptually illustrated in the flowchart of FIG. 10. In accordance withthis approach, a known force is applied in the vicinity of a touchsensor 1010. The force response of each force sensor is measured 920.The process of applying a known force at a sensor 910 and measuring theresultant response from each sensor 1020 is repeated until a known forcehas been applied in the vicinity of each of n touch sensors 1030.

An n×n data matrix M_(DATA) _(—) _(B) may be formed from the responsesof the n touch sensors to the known forces applied at each of n touchsensors 1040. A vector representing the total force {right arrow over(D)}_(ZB) the Y-axis moment {right arrow over (D)}_(YB) and the X-axismoment {right arrow over (D)}_(XB) may be formed from the known forcevalues 1050. The data matrix M_(DATA) _(—) _(B) may then be inverted1060 to form M_(DATA) _(—) _(B) ⁻¹. The calibration vectors {right arrowover (C)}_(ZB), {right arrow over (C)}_(YB), and {right arrow over(C)}_(XB) may be calculated 1070 as the dot products of the inverteddata matrix M_(DATA) _(—) _(B) ⁻¹ and the calculated total force {rightarrow over (D)}_(ZB), y-axis moment {right arrow over (D)}_(YB), and thex-axis moment {right arrow over (D)}_(XB), respectively.

In an exemplary embodiment of the above-described method, a touchsurface of a four-sensor unit under test is oriented horizontally. Aknown test weight is then placed on the touch surface such that itscenter of gravity falls over each of four known points in succession.These points may be chosen to fall close to the corner located sensors,but inset somewhat to avoid edge interferences. For instance, they maybe chosen to fall at the corners of a centered rectangle 15% smallerthan the touch surface itself. The weight, or each of four identicalweights, may be placed with the aid of a fixture or automatic apparatus.

Four sets of test data are collected, each comprising a vector of fourdifferences between the sensor readings for a particular applicationminus those with no weight applied. Each of these data vectors is thenexpected, when dotted with the calibration vector for totalperpendicular force, to yield the known test weight value (or someconvenient scaling thereof). Equivalently, a 4×4 data matrix may beformed from the data vectors in order as rows. This is expected, whenmultiplied on the right by the calibration vector for totalperpendicular force, to yield an expected force vector of fourcomponents all equal to the test force. Thus, the calibration vector fortotal perpendicular force may be extracted by multiplying the inverse ofthe data matrix by this expected force vector on the right.

Similarly, there is a vector of expected moments about the desiredY-axis. These moments are equal to the X-position of each test point inorder times the known weight value. The calibration vector for Y-axismoment may be extracted by multiplying the inverse of the data matrix bythis expected Y-axis moment vector on the right.

Similarly, there is a vector of expected moments about the desiredX-axis. These moments are equal to the Y-position of each test point inorder times the known weight value. The calibration vector for X-axismoment may be extracted by multiplying the inverse of the data matrix bythis expected X-axis moment vector on the right.

For example, calibration forces A, B, C, and D, of 150 grams-force each,may be applied at the following points:

-   -   Test force, upper left: X_(A)=−0.70 Y_(A)=0.70 F_(A)=150 gm.    -   Test force, upper right: X_(B)=0.70 Y_(B)=0.70 F_(B)=150 gm.    -   Test force, lower left: X_(C)=−0.70 Y_(C)=−0.70 F_(C)=150 gm.    -   Test force, lower right: X_(D)=0.70 Y_(D)=−0.70 F_(D)=150 gm.        The changes in sensor outputs occasioned by the applications of        these forces may be collected together as follows, both as        measurement vectors:        {right arrow over (F)}_(A)=[f_(A1), f_(A2), f_(A3), f_(A4)]        {right arrow over (F)}_(B)=[f_(B1), f_(B2), f_(B3), f_(B4)]        {right arrow over (F)}_(C)=[f_(C1), f_(C2), f_(C3), f_(C4)]        {right arrow over (F)}_(D)=[f_(D1), f_(D2), f_(D3),        f_(D4)]  [15]        and as a data matrix: $\begin{matrix}        {M_{DATA\_ B} = \begin{bmatrix}        f_{A\quad 1} & f_{A\quad 2} & f_{A\quad 3} & f_{A\quad 4} \\        f_{B\quad 1} & f_{B\quad 2} & f_{B\quad 3} & f_{B\quad 4} \\        f_{C\quad 1} & f_{C\quad 2} & f_{C\quad 3} & f_{C\quad 4} \\        f_{D\quad 1} & f_{D\quad 2} & f_{D\quad 3} & f_{D\quad 4}        \end{bmatrix}} & \lbrack 16\rbrack        \end{matrix}$        The vectors of expected forces and moments may be similarly        collected in the same A, B, C, D order: $\begin{matrix}        {{{\overset{\rightharpoonup}{D}}_{ZB} = \begin{bmatrix}        150 \\        150 \\        150 \\        150        \end{bmatrix}}{{\overset{\rightharpoonup}{D}}_{YB} = \begin{bmatrix}        {{- 0.70} \times 150} \\        {0.70 \times 150} \\        {{- 0.70} \times 150} \\        {0.70 \times 150}        \end{bmatrix}}{{\overset{\rightharpoonup}{D}}_{XB} = \begin{bmatrix}        {0.70 \times 150} \\        {0.70 \times 150} \\        {{- 0.70} \times 150} \\        {{- 0.70} \times 150}        \end{bmatrix}}} & \lbrack 17\rbrack        \end{matrix}$        The unknown calibration vectors must render the known data        matrix into the known forces and moments in accordance with:        M _(DATA) _(—) _(B) ·{right arrow over (C)} _(ZB) ={right arrow        over (D)} _(ZB)        M _(DATA) _(—) _(B) ·{right arrow over (C)} _(YB) ={right arrow        over (D)} _(YB)        M _(DATA) _(—) _(B) ·{right arrow over (C)} _(ZB) ={right arrow        over (D)} _(ZB)  [18]

Each of these three matrix equations corresponds to a system of fourlinear equations with four scalar unknowns. Among other methods, theunknown calibration vectors may be determined using the inverse of theknown matrix M_(DATA) _(—) _(B):{right arrow over (C)} _(ZB) =M _(DATA) _(—) _(B) ⁻¹ ·{right arrow over(D)} _(ZB){right arrow over (C)} _(YB) =M _(DATA) _(—) _(B) ⁻¹ ·{right arrow over(D)} _(ZB)  [19]{right arrow over (C)} _(XB) = _(DATA) _(—) _(B) ⁻¹ ·{right arrow over(D)} _(XB)

This method of calibration, as so far described, provides a basiccalibration with no provision for especially low susceptibility totorsional error. Indeed, with a rigid touch surface structure, variableloading of the sensors in a torsional pattern may be poorly representedin the data matrix M_(DATA) _(—) _(B). The susceptibility of theresulting calibration to torsional error may then be especially high.These problems may be addressed by applying the torsion responsecorrections as described above to the basic calibration vectors {rightarrow over (C)}_(ZB),{right arrow over (C)}_(YB), {right arrow over(C)}_(XB), or through further embodiments of the invention, such asthose described below.

In another embodiment of the method of the invention, illustrated in theflowchart of FIG. 11, torsion corrected calibration vectors are computedfrom data responsive both to known touch surface forces and todeliberately applied or enhanced torsional distortion. By this method,data matrix M_(DATA) _(—) _(B) is acquired 1110 by the method discussedimmediately above. A first distortion is applied to the touch screen anda first set of force response measurements obtained 1120. A seconddistortion is applied and the resulting second set of force responsivemeasurements obtained 1130. The differences, f_(Q1), f_(Q2), f_(Q3),f_(Q4), between the force response measurements resulting from thesecond applied distortion, f₁(t_(Q2)), f_(Q2)(t_(Q2)), f_(Q3)(t_(Q2)),f_(Q4) (t_(Q2)), and the first applied distortion, f₁(t_(Q1)),f_(Q2)(t_(Q1)), f_(Q3)(t_(Q1)), f_(Q4)(t_(Q1)), are calculated 1140.These values are used to extend the data matrix M_(DATA) _(—) _(B) by afifth row 1150 to form a torsion extended data matrix M_(DATA) _(—)_(T). The pseudo-inverse of the 5×4 data matrix M_(DATA) _(—) _(T) isdetermined 1160. The total expected force vector, Y-axis moment vector,and the X-axis moment vectors are calculated from known forces andcoordinates 1170. The torsion-corrected calibration vectors {right arrowover (C)}_(ZT), {right arrow over (C)}_(YT), {right arrow over (C)}_(XT)are then calculated as the as the products of the pseudo-inverseM_(DATA) _(—) _(T) ^(PSEUDO−1) of the extended data matrix M_(DATA) _(—)_(T) times the calculated total force {right arrow over (D)}_(ZT),Y-axis moment {right arrow over (D)}_(YT), and the X-axis moment {rightarrow over (D)}_(XT), respectively 1180.

By this method, the data matrix for basic calibration is extended byaddition of a fifth row. This row may comprise the response vector totorsion (or some linear scaling of it): $\begin{matrix}{M_{DATA\_ T} = \begin{bmatrix}f_{A\quad 1} & f_{A\quad 2} & f_{A\quad 3} & f_{A\quad 4} \\f_{B\quad 1} & f_{B\quad 2} & f_{B\quad 3} & f_{B\quad 4} \\f_{C\quad 1} & f_{C\quad 2} & f_{C\quad 3} & f_{C\quad 4} \\f_{D\quad 1} & f_{D\quad 2} & f_{D\quad 3} & f_{D\quad 4} \\f_{Q\quad 1} & f_{Q\quad 2} & f_{Q\quad 3} & f_{Q\quad 4}\end{bmatrix}} & \lbrack 20\rbrack\end{matrix}$

This extended 5×4 matrix may be multiplied on the right by each of thecalibration vectors sought:M _(DATA) _(—) _(T) ·{right arrow over (C)} _(ZT)M _(DATA) _(—) _(T) ·{right arrow over (C)} _(YT) ={right arrow over(D)} _(YT)M _(DATA) _(—) _(T) ·{right arrow over (C)} _(ZT) ={right arrow over(D)} _(ZT)  [21]

In each case, the first four elements of the resulting 5-element vectoron the right side should be the same as before, while the fifth elementis expected to be zero: $\begin{matrix}{{{\overset{\rightharpoonup}{D}}_{ZT} = \begin{bmatrix}150 \\150 \\150 \\150 \\0\end{bmatrix}}{{\overset{\rightharpoonup}{D}}_{YT} = \begin{bmatrix}{{- 0.70} \times 150} \\{0.70 \times 150} \\{{- 0.70} \times 150} \\{0.70 \times 150} \\0\end{bmatrix}}{{\overset{\rightharpoonup}{D}}_{XT} = \begin{bmatrix}{0.70 \times 150} \\{0.70 \times 150} \\{{- 0.70} \times 150} \\{{- 0.70} \times 150} \\0\end{bmatrix}}} & \lbrack 22\rbrack\end{matrix}$

While no inverse is defined for a 5×4 matrix, a suitable 4×5pseudo-inverse may be extracted by known methods employing its singularvalue decomposition. That is, a matrix M of m rows by n columns, m≧n,may in general be expressed as the product of three other matrices:M=U·W·V ^(T)  [23]where W is an n by n diagonal matrix, and U and V are column-orthonormalmatrices of sizes m by n and n by n, respectively. U, W, and V may befound by standard methods and an n by m pseudo-inverse expressed as:M _(DATA) _(—) _(T) ^(PSEUDO−1) =V·W ⁻¹ ·U ^(T)  [24]

Calibration coefficients may then be extracted by multiplying thispseudo-inverse on the right by each of the 5-element expected-resultvectors, in a manner analogous to that previously described for theconventional inverse. Note that the problem solved here is essentiallyone of achieving a best-fit solution to an overdetermined set of linearequations. Various methods to achieve the best-fit solution may be used.For example, solution by singular value decomposition withback-substitution may be computationally more efficient than use of anexplicitly formed pseudo-inverse.

In another such embodiment, varying torsion is applied to thetouch-screen support at the same time that known forces are applied tothe touch surface. Calibration coefficients are then determined from thedata matrix and the expected result vectors as described previously,although here there need not be any expected results that are zero, andthere need not be more than four data rows. Additional rows may be addedfor additional known force measurements if desired, however. Theoverdetermination may be handled as before. It will be evident to thoseof ordinary skill in the art that the method of the invention may beadapted to other procedures for extracting calibration coefficients,including those that employ a larger number of touch surface forcesapplied at known locations, but lacking known force values.

A first class of methods have been discussed, wherein a basiccalibration is prepared in one step, and refined with respect to torsionin another. This approach may offer the advantage of requiring lessunit-by-unit data measurement. It may work well for a certain range ofsuitable devices, including those with sensors of roughly similarsensitivity that are close to a rectangular pattern. It tends toeffectively minimize unwanted response to fluctuating torsion. On theother hand, there is the theoretical possibility that in the process, itmay “spoil” other aspects of the calibration, in the sense of degradingaccuracy in the absence of torsion. For suitable devices, however, thispotential problem is not significant.

A second class of methods have also been discussed, wherein atorsion-refined calibration is prepared in a single step. This approachmay offer the advantage of an optimized calibration over the full rangeof force-sensing touch location devices.

We now reconsider the case wherein the touch surface structure isrelatively rigid, in the sense that most of the small out-of-planemovements resulting from the application of a torsional force take placein the force sensors or the supporting structure. If the touch surfaceitself always remains plane, its motions in response to all test forcesmay explore only three degrees of freedom: slight vertical motions androtations, but no corner-to-corner saddling. Given this, calibrationonly from a set of known touch forces may remain an underconstrainedproblem, no matter how many forces and locations are used. Addingdeliberate variable torsion in the support resolves this problem.Without this, however, it is noted that sensitivity to torsionalinterference may be particularly high. In other words, a unit with arigid touch surface may be calibrated in the factory with a benignsupport lacking variable torsion. When that unit is placed in service inthe field, however, it may be vulnerable to large errors from variablesupport torsion. A method of the invention is thus particularlybeneficial in this case.

Such rigid touch surface devices may need only moderate torsionalexposure during calibration to achieve satisfactory results. In anotherembodiment of the invention, such moderate torsional exposure may beachieved by changing the relative compliance of the support under atleast one of the force sensors during the collection of calibrationdata. This may be accomplished in many ways. One approach involvesplacing materials made of differing compliance in the regions supportingthe different sensors, and then rotating the overall support surfaceafter half of the test forces have been applied.

A system for characterizing error in a touch screen associated withmechanical distortion is schematically illustrated in FIG. 12. In thisexample, a touch screen 1205 includes four touch sensors 1210, 1220,1230, 1240 located at four corners of a rectangular touch surface. Thetouch screen shown is the device for which mechanical distortion erroris to be characterized. Known forces may be applied to the touch screenat locations 1215, 1225, 1235, 1245 in the vicinity of the touch sensors1210, 1220, 1230, 1240 and the force response of the sensors measured inthe manner previously discussed. Additionally, one or more mechanicaldistortions of the touch screen may also be applied and the forceresponse measured. The touch sensors 1210, 1220, 1230, 1240 are coupledto a touch screen interface 1250 within the touch screen calibrationsystem 1260. The touch screen interface 1250 provides drive circuitryfor energizing the sensors, as well as sense circuitry for sensing forceresponsive touch signals from the sensors. The touch screen interfacedrive/sense circuitry is similar to the sensor drive/sense circuitry310, 320, 330, 340 schematically illustrated for the touch screencontroller FIG. 3. The touch screen interface 1250 is coupled to aprocessor 1264 within the touch screen calibration system. The processor1264 receives force responsive signals from the touch screen interface1250 and controls the processes of error characterization andcomputation of calibration parameters. The processor 1264 may be coupledto an output interface 1266 for recording or indicating the calibrationparameters 1270 determined by the touch screen calibration system 1260.The processor 1264 may also be coupled to memory circuitry 1262 forstoring program code and data, including calibration parameters, forexample.

It is to be appreciated that the calibration parameters 1270 may begrouped and represented in many different ways. Particular designs mayapply additional transformations of touch data. For instance, knownprocedures of “registration” may constitute an additional level ofadjustment, allowing a user-applied procedure to correct for varyingalignment of the touch screen with an underlying display raster. Such aprocedure may be combined with the calibration of the invention withoutdeparting from its scope, either by merging the required adjustment intothe calibration parameters of the invention, or by applying them in alater stage of calculation. Various computational arrangements may beused to apply torsion corrected calibration parameters along withparameters gathered for other purposes, without departing from the scopeof the invention.

A touch screen calibrated for reduced error from mechanical distortionas described herein may be advantageously implemented in various dataprocessing systems. Turning now to FIG. 13, a block diagram of a dataprocessing system 1300 using an integrated touch screen and display isshown in accordance with an embodiment of the present invention. Thesystem 1300 uses a transparent touch screen 1306 arranged above adisplay 1308 suitable for data processing applications, such as an LCDdisplay. Other displays may be used, such as a CRT display, plasmadisplay, LED display or the like. The display 1308 may require displaycontrol system circuitry 1309 for interfacing the display with the dataprocessor computer 1310. A touch screen control system 1307 includes thedrive/sense circuitry described above in addition to a touch screencontrol system processor according to an embodiment of the presentinvention.

The data processor 1310 may include various components depending uponthe computer system application. For example, the data processor mayinclude a microprocessor 1312, various types of memory circuitry 1314, apower supply 1318 and one or more input/output interfaces 1316. Theinput/output interfaces 1316 allow the data processing system to connectto any number of peripheral I/O devices 1320 such as keyboards 1321,pointing devices 1322, and sound devices 1323, including microphone andspeakers. The data processing system may additionally include a massdata storage device 1330, for example, a hard disk drive or CD ROMdrive, and may be networked to other data processing systems through aphysical or wireless network connection 1340.

FIG. 14 illustrates a touch screen system 1400 in accordance with thepresent invention, wherein the processes of the invention describedherein may be tangibly embodied in a computer-readable medium orcarrier, e.g. one or more of the fixed and/or removable data storagedevices 1410 illustrated in FIG. 14, or other data storage or datacommunications devices. One or more computer programs 1420 expressingthe processes embodied on the removable data storage devices 1410 may beloaded into various memory elements 1430 located within the touch screencontrol system 1440 to configure the touch screen system 1400 foroperation in accordance with the invention. The computer programs 1420comprise instructions which, when read and executed by the touch screensystem processor 1450 of FIG. 14, cause the touch screen system 1400 toperform the steps necessary to execute the steps or elements fordetecting the location of a touch on a touch screen in accordance withthe principles of the present invention.

FIG. 15 illustrates a touch screen calibration system 1500 in accordancewith the present invention, wherein the processes of the inventiondescribed herein may be tangibly embodied in a computer-readable mediumor carrier, e.g. one or more of the fixed and/or removable data storagedevices 1510 illustrated in FIG. 15, or other data storage or datacommunications devices. One or more computer programs 1520 expressingthe processes embodied on the removable data storage devices 1510 may beloaded into various memory elements 1530 located within the touch screencalibration system 1550 to configure the touch screen calibration system1550 for operation in accordance with the invention. The computerprograms 1520 comprise instructions which, when read and executed by thetouch screen calibration system 1550 of FIG. 15, cause the touch screencalibration system 1550 to perform the steps necessary to execute thesteps or elements for detecting the location of a touch on a touchscreen in accordance with the principles of the present invention.

A touch sensing method and system in accordance with the principles ofthe present invention provides for enhanced touch location accuracy inthe presence of mechanical distortions of the touch screen. Othermethods of improving touch location accuracy may be advantageouslycombined with the method of the present invention to further enhancelocation accuracy.

One method for timing the touch location measurement for enhanced touchlocation accuracy is described in U.S. patent application entitled“Method for Improving Positioned Accuracy for a Determined Touch Input,”identified under Docket Number 57470US002 (US publication number03-0206162-A1, published on Nov. 6, 2003), which is hereby incorporatedherein by reference in its entirety. According to this method, touchlocation may be calculated from data gathered at a preferred time withinthe touch signal time profile. The method of timing the touch locationmay be combined with calibration methods of the present invention tofurther improve the accuracy of a touch location determination.

Another method for improving touch location accuracy is described inco-owned U.S. patent application entitled “Improved BaseliningTechniques in Force-Based Touch Panel Systems,” identified under DocketNumber 57471US002 (US publication number 03-0210235-A1, published onNov. 13, 2003), which is hereby incorporated herein by reference in itsentirety. One or more reference levels may be identified for a touchsignal. The reference levels may compensate for various conditionsaffecting the touch screen at the time of the touch. Touch locationaccuracy may be further enhanced using one or more of the identifiedtouch signal reference levels for determining the touch location incombination with the calibration methods provided in the presentinvention.

Yet another method for improving touch location accuracy by correctingtouch signal errors associated with viscoelastic memory effects isdescribed in co-owned U.S. patent application entitled “Correction ofMemory Effect Errors in Force-Based Touch Panel Systems,” identifiedunder Docket Number 57472US002 (US publication number 03-0214486-A1,published on Nov. 20, 2003), which is hereby incorporated herein byreference in its entirety. Correction of touch signal errors associatedwith memory effects in combination with the calibration methods of thepresent invention may improve the accuracy of touch locationdetermination.

The touch sensing approach described herein is well-suited for use withvarious data processing systems, including personal data assistants(PDAs), electronic instruments, cell phones, and computers, includinghandheld, laptop and desktop computers.

The present invention should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the present specification. Theclaims are intended to cover such modifications and processes.

1. A method for calibrating a touch screen, comprising: applyingmechanical distortion to the touch screen; detecting a force responsivetouch signal arising from the mechanical distortion of the touch screen;characterizing a touch signal error associated with the mechanicaldistortion; and producing calibration parameters using thecharacterization of the touch signal error.
 2. The method of claim 1,wherein characterizing the touch signal error associated with themechanical distortion comprises: applying two or more distortionconditions to the touch screen; detecting sensor signals correspondingto each distortion condition; and characterizing the effect ofdistortion using the sensor signals corresponding to each distortioncondition.
 3. The method of claim 2, further comprising: determining oneor more basic calibration vectors; and determining one or moredistortion corrected calibration vectors using the one or more basiccalibration vectors and the sensor signals corresponding to eachdistortion condition.
 4. The method of claim 3, wherein determining theone or more basic calibration vectors comprises calculating the one ormore basic calibration vectors from nominal sensor locations and knownparameters of the touch screen design.
 5. The method of claim 3, whereindetermining the one or more basic calibration vectors comprisesmeasuring sensor locations and touch screen parameters.
 6. The method ofclaim 3, wherein determining the one or more basic calibration vectorscomprises measuring sensor signals corresponding to one or more knownforces applied at one or more known locations on the touch screen. 7.The method of claim 3, wherein determining the one or more basiccalibration vectors comprises applying known forces in the vicinity ofeach touch sensor location and measuring the sensor signals responsiveto the known forces.
 8. The method of claim 2, wherein applying the twoor more distortion conditions comprises: applying a zero distortion as afirst distortion condition; and applying a non-zero distortion as asecond distortion condition.
 9. The method of claim 2, wherein applyingthe two or more distortion conditions comprises: applying a firstnon-zero distortion condition as a first distortion condition; andapplying a second non-zero distortion condition as a second distortioncondition.
 10. The method of claim 9, wherein the first distortioncondition has an effect opposite to an effect of the second distortioncondition.
 11. The method of claim 2, wherein applying two or moredistortion conditions to the touch screen comprises applying thedistortion conditions to the touch screen at the same time that knownforces are applied to the touch screen.
 12. A touch screen calibrationsystem, comprising: a mechanical distortion system for applyingmechanical distortion to the touch screen; a detection system fordetecting force responsive sensor signals arising from the mechanicaldistortion; and a processor, coupled to the detection system, andreceiving the sensor signals detected by the detection system, theprocessor configured to detect a force responsive touch signal arisingfrom the mechanical distortion of the touch screen, characterize a touchsignal error associated with the mechanical distortion of the touchscreen, and produce calibration parameters using the characterization ofthe touch signal error.
 13. The system of claim 12 wherein themechanical distortion system is configured to apply two or moremechanical distortion conditions to the touch screen.
 14. The system ofclaim 12, wherein the mechanical distortion applied to the touch screenis torsion.
 15. The system of claim 13, wherein the processor isconfigured to detect sensor signals arising from each mechanicaldistortion condition, and characterize the touch signal error using thesensor signals arising from each mechanical distortion condition. 16.The system of claim 12, further comprises a force application system forapplying known forces to the touch screen, wherein the processor isfurther configured to determine one or more basic calibration vectors,and determine one or more distortion corrected calibration vectors usingthe one or more basic calibration vectors and the sensor signalscorresponding to each distortion condition.
 17. The system of claim 16,wherein the known forces are applied by the force application system inthe vicinity of each touch sensor location.
 18. The system of claim 13,wherein the mechanical distortion system: applies a zero distortion as afirst distortion condition; and applies a non-zero distortion as asecond distortion condition.
 19. The system of claim 13, wherein themechanical distortion system: applies a first non-zero distortioncondition as a first distortion condition; and applies a second non-zerodistortion condition as a second distortion condition.
 20. The system ofclaim 19, wherein the first distortion condition has an effect oppositeto an effect of the second distortion condition.
 21. The system of claim13, wherein the mechanical distortion system applies two or moredistortion conditions at the same time that known forces are applied tothe touch screen.
 22. A system for calibrating a touch screen,comprising: means for applying mechanical distortion to the touchscreen; means for detecting sensor signals associated with themechanical distortion; and means for calibrating the touch screen tocompensate for the mechanical distortion.
 23. The system of claim 22,wherein means for applying mechanical distortion to the touch screencomprises means for applying a first and a second torsion to the touchscreen.
 24. The system of claim 22, wherein means for calibrating thetouch screen comprises: means for determining a basic calibration; andmeans for calibrating the touch signal using the basic calibration andthe detected sensor signals affected by the mechanical distortion. 25.The system of claim 24, wherein means for determining a basiccalibration comprises: means for applying known forces to the touchscreen; means for detecting sensor signals responsive to the knownforces; and means for determining basic calibration vectors for thetouch screen using the sensor signals responsive to the known forces.26. The system of claim 24, wherein means for calibrating the touchsignal using the basic calibration and the detected sensor signalsaffected by the mechanical distortion comprises means for determiningtorsion corrected calibration vectors for the touch screen.
 27. Thesystem of claim 22, wherein means for applying mechanical distortion tothe touch screen comprises means for applying the distortion conditionsto the touch screen at the same time that known forces are applied tothe touch screen.
 28. A computer-readable medium configured withexecutable instructions for causing one or more computers to perform amethod of calibrating a touch screen, the method comprising: applyingmechanical distortion to the touch screen; detecting a force responsivetouch signal arising from the mechanical distortion of the touch screen;characterizing a touch signal error associated with the mechanicaldistortion, the touch signal error arising in a force responsive touchsignal; and producing calibration parameters using the characterizationof the touch signal error.